The US patent U.S. Pat. No. 6,091,547 (3M, 2000) discloses the use of a prismatic film, which collects light from ‘off-axis’ and redirects this light ‘on-axis’ and which is known as a brightness enhancement film (BEF). A schematic of this film is shown in FIG. 1A of the accompanying drawings. Light produced by the source can be efficiently directed towards the viewer resulting in a brighter display. This film cannot be used efficiently with a curved backlight, as uniformity is not maintained when this film is curved. In particular, the direction of brightness enhancement becomes different when the film is curved. Typically BEFs are used as an orthogonal pair inside a backlight, in order to condense the light in both directions.
The Japanese patent JP 2004/288570 (Toshiba) discloses curved displays, where the distribution of the extraction features in the lightguide is modified to allow on axis uniformity, though it does not describe in detail the scatter points or their distribution.
The US patent U.S. Pat. No. 5,940,215 (Ericsson, 1998) discloses a flexible light-guide whereby microstructure is moulded onto the surface as a diffuser so that the emission from the light-guide is more even and broad. However, the display is relatively dim to an on-axis viewer.
The US patent U.S. Pat. No. 7,221,847 (3M) discloses a flat optical element, such as a lightguide or an optical film, which is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output of a lightguide, or may be arranged to otherwise enhance the performance of a display.
The US patent U.S. Pat. No. 6,752,505 (Solid State Opto Limited) discloses light redirecting films, which include a pattern of individual optical elements of well defined shape on the light exit surface of the films for refracting the light entering the entrance surface of the films from a backlight toward a direction that is normal to the exit surface. The individual optical elements overlap and intersect each other. Also, the orientation, size and/or shape of the optical elements may be tailored to redirect more of the incident light from the flat backlight within a desired viewing angle. An example of such an arrangement is shown in FIG. 1B of the accompanying drawings.
The US patent application no. 2005/0024754 (3M) discloses a film where prism structures are metalized and reshaped in order to reflect incident light within a desired reflective light pattern. Such a film is designed to redirect light of a reflective flat display and an example is shown in FIG. 1C of the accompanying drawings.
The U.S. patent application Ser. No. 11/301995 discloses a micro-structured optical film used to enhance on-axis luminance and remove windscreen reflections in cars. Such a film is designed for a flat backlight.
The British patent application no. 2443849 discloses wedge shaped extraction features in the lightguide to correct direction in the curve from a curved lightguide.
FIG. 2 of the accompanying drawings illustrates a typical display of known type that is used in common Liquid Crystal Displays. The display is composed of a flat transmissive spatial light modulator (SLM) in the form of a liquid crystal display (LCD) panel 2 placed between polarizers 3 and 4. The panel is provided with a backlight 12, whose main components are diffusers 5 and 6, a pair of BEF's, 1, arranged in an orthogonal fashion to form horizontal and vertical BEF's, a light guide 8 with its extraction features 9 (exaggerated in this diagram), one or more illumination devices 10 and a rear reflector film 11. The light from the source is coupled into a light guide 8, in which total internal reflection (TIR) takes place so that the light would travel along the light guide until it reached the far end if there were no scattering structure in the light guide. However, within the light guide 8 there are many extraction features 9 that frustrate the TIR and couple the light out of the light guide to illuminate the LCD panel. The rear reflector film 11 under the light guide 8 increases the out-coupling efficiency. There are also some optical films between the light guide 8 and LCD panel 2. Diffusers 5, 6 and BEF's 1 are present to achieve better illumination uniformity and to increase the brightness within a certain viewing angle range.
Some other films (not shown), such as a DBEF, could also be applied to such a backlight.
FIG. 3 of the accompanying drawings shows the display of FIG. 2 curved and comprising a curved LCD, 13, and a backlight 14. The radius of curvature in this case is much less than the viewing distance of the display viewer.
When using a standard curved backlight as shown in FIG. 4A of the accompanying drawings, a shift in the angular distribution, 22, is seen along the backlight unit as shown in FIG. 4B of the accompanying drawings. This is because the angle at which the viewer sees a point on the display, 20, now varies relative to the local display normal, 21. As a result, a viewer looking at this backlight will see an apparently non-uniform backlight, 24, with brighter and dimmer parts as illustrated in FIG. 4C of the accompanying drawings.
BEF's condense the light in such a way as to enhance the light within a certain angular range but, when bent, the prism-like structures change direction along the curve as shown in FIG. 5 of the accompanying drawings. As the BEF enhances light along the local normal, 21, and not along the viewer direction, 20, the apparent brightness of the display will also be reduced over the flat case.
Three main ways have been followed in the past to solve this problem of non-uniformity.
One comprises changing the distribution of the extraction features 9. By changing this distribution, the total amount of light extracted by the lightguide becomes non-uniform. Nevertheless, with such a method, it is possible to make brighter some parts of the lightguide and dimmer some others, allowing a certain control of the uniformity within a predetermined direction but being non-uniform in the others. However, this does not address the fundamental problem of direction and the viewing freedom is limited in this type of display, especially at lower bend radii.
A second way comprising using a strong diffuser has been followed, giving a uniform backlight unit by spreading the light. This however does not enhance brightness in any particular direction and the display will appear dimmer than an existing flat display.
A third is by adapting the extraction features 9 to control the light output direction to maintain uniformity and viewing freedom. These methods entail designing a complete lightguide for each different curve. In addition, the extraction features may be difficult to manufacture.